The determination of acetaldehyde in biologic samples is complicated by a variety of formation and disappearance reactions occurring in the present methods of acetaldehyde analyses. The acetaldehyde formation (ethanol oxidation) in deproteinized supernatant of tissue preparations is prevented by the use of thiourea. During deproteinization, however, it is not inhibited by thiourea, and this remains the main problem in blood acetaldehyde determinations. To circumvent this problem, the use of a correction curve is proposed which is generated by adding control blood samples to the deproteinizing agent such that the blood dilution, temperature, and the ethanol concentrations (the main factors affecting the artifactual acetaldehyde formation) in the controls are identical to those of the samples. Disappearance reactions mainly include loss of acetaldehyde due to binding and/or metabolism. The problem seems to be pronounced with human blood samples, and it is recommended that they be rapidly (<5 sec) deproteinized.